Download Ch 20 Electrochemistry:

Ch 20 Electrochemistry:
the study of the relationships between
electricity and chemical reactions.
In electrochemical reactions, electrons are
transferred from one species to another.
Learning goals and key skills:
Identify oxidation, reduction, oxidizing agent, and reducing agent in a chemical equation
Complete and balance redox equations using the method of half-reactions.
Sketch a voltaic cell and identify its cathode, anode, and the directions that electrons and
ions move.
Calculate standard emfs (cell potentials), Eocell, from standard reduction potentials.
Use reduction potentials to predict whether a redox reaction is spontaneous
Relate Eocell to ∆Go and equilibrium constants.
Calculate emf under nonstandard conditions
Describe to reactions in electrolytic cells
Relate the amounts of products and reactants in redox reactions to electrical charge.
Review redox reactions:
assigning oxidation numbers
To keep track
of what species
loses electrons and
what species gains
them, we assign
oxidation numbers.
Assigning oxidation numbers
How to determine oxidation numbers
1. Elemental form, each atom has ox. # = 0.
Zn
O2
O3
I2
S8
P4
2. In simple ions, ox. # = charge on ion.
-1 for Cl+2 for Mg2+
3. The ox. # of F is ALWAYS -1 with all other
elements.
FeF3
PF5
SF6
OF2
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Assigning oxidation numbers
4. The ox. # of O is normally -2 (except in the
peroxide ion, which has an oxidation number
of −1) and the ox. # of H is +1 (except with
metals).
O is -2 in NO
H is +1 in CH4
O is -1 in Na2O2
H is -1 in CaH2
5. Algebraic sum of oxidation numbers
= 0 for a compound
= overall charge for an ion
Example
Assign the oxidation numbers to each element:
SiBr4
K3PO4
Cu(NO3)3
Redox reactions
ELECTRON TRANSFERS between an electron donor
and electron acceptor.
Transfer leads to—
1. increase in oxidation number = OXIDATION
2. decrease in oxidation number = REDUCTION
Oxidizing agent or oxidant: causes the oxidation
Reducing agent or reductant: causes the reduction
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Example
Label the species that undergo oxidation and
reduction. Also identify the oxidizing agent and
reducing agent.
Cu (s) + 2 AgNO3 (aq) → Cu(NO3)2 (aq) + 2 Ag (s)
Example
Label the species that undergo oxidation and
reduction. Also identify the oxidizing agent and
reducing agent.
Reduction
Oxidation
Cu (s) + 2 AgNO3 (aq) → Cu(NO3)2 (aq) + 2 Ag (s)
0
+1 +5 -2
+2 +5 -2
0
Oxidizing agent: Ag in AgNO3; Reducing agent: Cu
Electron transfer reactions
Essential features:
• one reactant is oxidized
• one reactant is reduced
• the oxidizing agent is reduced
• the reducing agent is oxidized
• an element is oxidized if its ox # increases;
it is reduced if its ox # decreases.
• the extent of oxidation and reduction must balance
All redox reactions must be balanced for
both mass and charge.
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Steps for balancing equations
by the method of half-reactions
1. Assign oxidation states.
2. Divide the equation into two half-reactions.
3. Balance each half-reaction as follows:
a. Balance elements other than O and H
b. Balance O by adding H2O
c. Balance H by adding H+
d. Balance charge by adding eto the more positive side
4. Multiply by factors so that e- lost = e- gained
5. Add and cancel (check mass & check charge).
6. If in a basic solution, add OH- to neutralize.
Balancing redox reactions:
using half-reactions
Cu (s) + AgNO3 (aq) → Cu(NO3)2 (aq) + Ag (s)
In the above unbalanced reaction, the spectator ion
(NO3-) is unimportant to the net equation, so let’s
write the unbalanced net equation.
Cu (s) + Ag+ (aq) → Cu2+ (aq) + Ag (s)
Balancing redox reactions:
using half-reactions
Cu (s) + Ag+ (aq) → Cu2+ (aq) + Ag (s)
There are two halves of the reaction: the oxidation
half and the reduction half.
Cu (s) → Cu2+ (aq) + 2 eAg+ (aq) + e- → Ag (s)
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Balancing redox reactions:
using half-reactions
Balance the charges and masses to make the full
redox reaction
Cu (s) → Cu2+ (aq) + 2 e2 Ag+ (aq) + 2 e- → 2 Ag (s)
Cu (s) + 2 Ag+ (aq) → Cu2+ (aq) + 2 Ag (s)
Sometimes it’s necessary to add H2O and either H+ or
OH-.
Half-Reaction Method Example
Consider the reaction between MnO4− and C2O42− :
MnO4− (aq) + C2O42− (aq) → Mn2+ (aq) + CO2 (g)
First, we assign oxidation numbers.
+7
+3
+2
+4
MnO4− + C2O42- → Mn2+ + CO2
Since the manganese goes from +7 to +2, it is reduced.
Since the carbon goes from +3 to +4, it is oxidized.
Oxidation Half-Reaction
C2O42− → CO2
To balance the carbon, we add a coefficient of 2:
C2O42− → 2 CO2
The oxygen is now balanced as well. To balance
the charge, we must add 2 electrons to the right
side.
C2O42− → 2 CO2 + 2 e−
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Reduction Half-Reaction
MnO4− → Mn2+
The manganese is balanced; to balance the oxygen, we
must add 4 waters to the right side.
MnO4− → Mn2+ + 4 H2O
To balance the hydrogen, we add 8 H+ to the left side.
8 H+ + MnO4− → Mn2+ + 4 H2O
To balance the charge, we add 5 e− to the left side.
5 e− + 8 H+ + MnO4− → Mn2+ + 4 H2O
Combining the Half-Reactions
Now we evaluate the two half-reactions together:
C2O42− → 2 CO2 + 2 e−
5 e− + 8 H+ + MnO4− → Mn2+ + 4 H2O
To attain the same number of electrons on each side, we will
multiply the first reaction by 5 and the second by 2.
5 C2O42− → 10 CO2 + 10 e−
10
+ 16 H+ + 2 MnO4− → 2 Mn2+ + 8 H2O
When we add these together, we get:
10 e− + 16 H+ + 2 MnO4− + 5 C2O42− →
2 Mn2+ + 8 H2O + 10 CO2 +10 e−
16 H+ + 2 MnO4− + 5 C2O42− →
2 Mn2+ + 8 H2O + 10 CO2
e−
Example
Iodide will react with permanganate ions to
form iodine and manganese (IV) oxide. Write
the balanced net ionic equation if the reaction
occurs in an acidic solution.
6 I- (aq) + 2 MnO4- (aq) + 8 H+ (aq)
3 I2 (aq) + 2 MnO2 (aq) + 4 H2O (l)
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Example
Do the previous example in a basic solution.
Iodide will react with permanganate ions to
form iodine and manganese (IV) oxide.
If a reaction occurs in basic solution, one can
balance it as if it occurred in acid.
Once the equation is balanced, add OH− to each
side to “neutralize” the H+ in the equation and
create water in its place.
If this produces water on both sides, you might
have to subtract water from each side.
Two types of electrical cells
Voltaic/galvanic cell
spontaneous chemical reaction
that produces electrical energy
Electrolytic cell
electrical energy is used for a
nonspontaneous chemical reaction
Voltaic cells
•In spontaneous
oxidation-reduction
(redox) reactions,
electrons are
transferred and
energy is released.
•We can use that
energy to do work if
we make the
electrons flow
through an external
device.
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Voltaic cells
The oxidation occurs at the anode.
The reduction occurs at the cathode.
Voltaic cells description
When electrons flow
from the anode to
the cathode, the
charges in each
beaker would not
be balanced and
the flow of
electrons would
stop.
Voltaic Cells
Therefore, we use a
salt bridge, usually
a U-shaped tube
that contains a salt
solution, to keep
the charges
balanced.
Cations move
toward the
cathode.
Anions move
toward the anode.
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Voltaic Cells
•In the cell, electrons
leave the anode and flow
through the wire to the
cathode.
•As the electrons leave the
anode, the cations formed
dissolve into the solution
in the anode
compartment.
•As the electrons reach
the cathode, cations in the
cathode are attracted to
the now negative cathode.
•The electrons are taken
by the cation, and the
neutral metal is deposited
on the cathode.
anode
(oxidation)
cathode
(reduction)
Zn (s) | Zn2+ (aq)
||
Cu2+ (aq) | Cu (s)
Electromotive force (emf)
Water spontaneously
flows one way in a
waterfall.
Likewise, electrons
only spontaneously
flow one way in a
redox reaction —
from higher to lower
potential energy.
Electromotive force (emf)
The potential difference between the
anode and cathode in a cell is called the
electromotive force (emf).
It is also called the cell potential (Ecell).
Cell potential is measured in volts (V)
and it is often called the cell voltage.
1V=1
J
C
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Standard cell potentials
The cell potential at standard conditions
can be found through this equation:
Ecell
° (cathode) − Ered
° (anode)
° = Ered
Because cell potential is based on the
potential energy per unit of charge, it
is an intensive property.
Standard Hydrogen Electrode
2 H+
(aq, 1M)
+ 2 e− → H2
(g, 1 atm)
By definition, the reduction potential for hydrogen is 0 V:
Using a SHE
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Cell Potentials
For the oxidation in this cell,
° = −0.76 V
Ered
For the reduction,
° = +0.34 V
Ered
° = Ered
° (cathode) −Ered
° (anode)
Ecell
= +0.34 V − (−0.76 V) = +1.10 V
Standard reduction potentials
-0.73
Cr3+(aq) + 3e-
Cr(s)
Standard reduction potentials
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Oxidizing and reducing agents
The greater the
difference between the
two, the greater the
voltage of the cell.
E°cell > 0
product favored
E°cell < 0
reactant favored
Standard reduction potentials
1. All potentials are for reduction reactions.
2. The more positive E°, the better the oxidizing ability.
3. The more negative E°, the more likely the reverse
(oxidation) reaction will occur.
4. When the reaction is reversed, the sign changes.
5. NW-SE rule: reaction of the upper left with the lower
right is product favored.
6. The sign of the half-reaction potential is in reference to the
SHE (H2 (g, 1 bar)/H+ (aq, 1 M) standard cell).
7. Potentials depend on the nature of reactants and products and
NOT on the amounts.
Example
Draw the voltaic cell, use cell notation to describe the
voltaic cell, and calculate the standard EMF for a cell
containing cadmium and silver electrodes.
Ag+ (aq) + e- → Ag (s)
E° = +0.799 V
Cd2+
E° = -0.40 V
(aq) + 2
e-
→ Cd (s)
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Free Energy
∆G for a redox reaction:
∆G = −nFE
Under standard conditions,
∆G° = −nFE°
where n is the number of moles of electrons
transferred, and F is the Faraday constant.
1 F = 96,485 C/mol = 96,485 J/V-mol
Example
Calculate ∆G° for
2 Cr(s) + 3 Pb2+(aq)
2 Cr3+(aq) + 3 Pb(s)
from Appendix E and supplemental sources
Cr3+ (aq) + 3 e- → Cr
(s)
E° = -0.73 V
Pb2+ (aq) + 2 e- → Pb (s)
E° = -0.126 V
Nernst Equation
∆G = ∆G° + RT ln Q
This means
−nFE = −nFE° + RT ln Q
Dividing both sides by −nF, we get
the Nernst equation:
E = E° −
RT
nF
ln Q
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Nernst equation:
cell potentials at nonstandard conditions
E = E° −
RT
nF
ln Q
E = E° −
2.303 RT
log Q
nF
E = E° −
0.0592
log Q
n
at 25 °C
Example
Calculate the emf for the following cell at 25 °C.
Cr (s) | Cr3+ (aq, 1.00 M)
||
Pb2+ (aq, 0.010 M) | Pb (s)
Example
Calculate the emf for the following reaction at 25 °C.
4 Fe2+(aq) + O2(g) + 4H+(aq) → 4 Fe3+(aq) + 2H2O(l)
a) Under standard conditions
b) When [Fe2+] = 1.3 M, [Fe3+] = 0.010 M, PO2 = 0.50 atm, and
the pH in the cathode half-cell is 3.50.
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Concentration cells
A cell could be created that has the same substance
at both electrodes.
• E°cell would be 0, but Q would not be.
• E will not be 0 as long as the concentrations are
different.
Applications
Hydrogen fuel cells
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Hydrogen fuel cell
Adsorbing H2 onto a metal or metal alloy.
Example
Determine the amount of voltage that is necessary to
convert water into hydrogen and oxygen gas.
2 H2O (l) + 2 e-
H2 (g) + 2 OH- (aq)
O2 (g) + 2 H2O (l) + 4 e-
4 OH- (aq)
E°red = -0.8277 V
E°red = +0.40 V
Corrosion and…
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…Corrosion Prevention
…Corrosion Prevention
Electrolytic cells
inert electrode
inert electrode
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Stoichiometry of Electrolysis
2 mol e-: 1 mol Cu
1 A = 1 C/s
Current(C/s) ×
time(s) =
charge(C)
F=
96,485 C
mole e-
A voltaic/galvanic cell:
spontaneous
+
cathode
anode
An electrolytic cell:
nonspontaneous
+
anode
cathode
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